The U.K. service sector growth strengthened more-than-expected in May but the pace of expansion was one of the weakest seen over the past three years, survey data from Markit Economics showed Friday. The Markit/Chartered Institute of Procurement & Supply Services Purchasing Managers’ Index climbed to 53.5 from a 38-month low of 52.3 in April. The score was forecast to rise marginally to 52.5.